Mechanism of short-chain alkane transformation over protonic zeolites. Alkylation, disproportionation and aromatization

Abstract

Ethane, propane and butanes can be transformed on large and average pore size protonic zeolites. It is shown that the reaction temperature determines both the mode of alkane activation on the acid sites (i.e. the mode of carbocation formation) and the nature of their transformation. At ambient temperature isobutane (and no other short-chain alkanes) can be activated, but only in the presence of alkenes. This activation occurs by hydride transfer to the carbenium ions resulting from alkene adsorption on the protonic sites of the zeolite. The t-butyl carbenium ions, formed from isobutane, alkylate the alkene molecules through a chain mechanism. At temperatures above 500°C, pure C 2–C 4 alkanes can be transformed into aromatics, in particular on HMFI. The activation of alkanes occurs, like in superacid solutions, through protolysis of their C H or C C bonds with formation of hydrogen or alkanes and of carbenium ions which desorb as olefins. These olefins are transformed into aromatic products through various reactions: oligomerization-cracking cyclization and hydrogen transfer. On HMFI, the protolytic cleavage of C H and C C bonds is the limiting step of short-chain alkane aromatization. The association of gallium species to HMFI increases significantly the aromatization activity and selectivity of this zeolite and alkane aromatization occurs through a bifunctional scheme. At average temperatures, propane and butanes can be transformed through a dimerization-cracking process (disproportionation). In butane transformation this process is responsible for the formation of propane and pentanes but also for butane isomerization. As is the case at high temperature the activation of alkanes occurs through protolysis. However this protolysis is only the initiation step of the carbenium ion chain mechanism of disproportionation, for protolysis is much slower than the hydride transfer from alkanes to the resulting carbenium ions. The reaction schemes of isobutane alkylation with 2-butene, of butane isomerization and of propane aromatization are described. The influence of the pore structure and of the acidity of the protonic zeolites on their activity and selectivity is discussed.